1. Field of the Invention
The present invention relates to a bi-directional wavelength switching device and a wavelength demultiplexing/multiplexing device suitable for use in the wavelength division multiplexed transmission system.
2. Description of the Related Art
Accompanied with the recent advanced developments and intricacies in communication technology, wavelength division multiplexed ("WDM") transmission has been proposed as a way to transmit large amounts of information on optical fibers. FIG. 20 is a block diagram generally illustrating a proposed wavelength division multiplexed transmission system. The WDM transmission system 100' shown in FIG. 20 employs wavelength demultiplexing/multiplexing devices 1'-a and 1'-b to be integrated into a WDM network.
The transmission line usually employs more than one pair of optical fibers 7'. One pair will be considered. The other pairs may provide for additional information transmission or provide for backups. One of the optical fibers 8'-a in the pair is used for the upstream communication line, and another optical fiber 8'-b is used for the downstream communication line. Optical amplifier repeaters 9'-a are placed in order to compensate for losses in the optical fibers 8'-a and 8'-b on the upstream and downstream communication lines. One optical amplifier repeater 9'-a is provided with at least two optical amplifiers 9'-b (more than two amplifiers for more fibers) for the upstream and downstream communication lines. From each of the terminal stations 50a', 50b', 50c', and 60', a plurality of optical signals (WDM signals) respectively having different wavelengths are transmitted into one optical fiber. The WDM signals are split into the various transmission lines according to wavelength by the wavelength demultiplexing/multiplexing devices 1'-a and 1'-b to thereby be transmitted to the terminal stations 50a', 50b', 50c', and 60'.
The wavelength demultiplexing/multiplexing devices 1'-a, 1'-b used for the WDM network each include a combination of OADM (optical add-drop multiplexer) circuits.
FIG. 21 is a chart to explain the basic character of an OADM circuit. The OADM circuit 30'a drops only the optical signals having selected wavelengths from the WDM signals having a plurality of wavelengths (.lambda.1, .lambda.2, ..., .lambda.n) propagating in a trunk system transmission fiber 8'-c. These optical signals are dropped to a drop transmission fiber 8'-e. The OADM circuit 30a' adds optical signals input from an add transmission fiber 8'-d to the optical signals travelling on trunk system fiber 8'-c. The added optical signals and the signals not dropped are output onto a trunk system transmission fiber 8'-f. Usually, the same wavelength is selected for the wavelength of the optical signal to be dropped and the wavelength of the optical signal to be added.
In the WDM optical communication system, normally one or more optical fiber pairs are used for the upstream and downstream transmission lines. Accordingly, the wavelength demultiplexing/multiplexing devices 1'-a and 1'-b are comprised of more than two of the OADM circuits shown in FIG. 21. The wavelength demultiplexing/multiplexing device 1'-a (1'-b) is constructed such that an OADM circuit 30'a intervenes in each trunk system optical fiber 8'-a and 8'-b, with each OADM circuit connected to a separate drop and add optical fibers 8-g and 8-h, as shown in FIG. 22.
Further, to give the OADM circuit the capability of selecting the wavelength to be dropped or added, it is conceivable to use an acoustic-optic tunable filter (hereunder, referred to as "AOTF") capable of varying the permeability for the OADM circuit. The AOTF is a device in which an acousto-optical effect is applied, which can be used effectively as an optical filter that can vary the filtered wavelength. The construction of the AOTF has been proposed in several types, however, the basic operational principle is the same.
FIG. 23 shows an example of an AOTF. The AOTF 30' employs a radio frequency ("RF") signal, which is input to an electrode 30'-1 (IDT, hereunder referred to as a transducer) through a control port 30-7 to thereby produce a surface acoustic wave ("SAW"). The SAW propagates in an SAW cladding 30'-2, and is absorbed by an SAW absorber 30'-3. On the other hand, the optical signals come in from an optical input port 01, and are polarized and split by a Polarization Beam Splitter ("PBS") 30'-4 into two optical waveguides. The SAW and the optical signals overlap and interfere, to polarize only the optical signals having a wavelength corresponding to the frequency of the SAW. This is due to the acousto-optical effect. The selectively polarized optical signals are split off by a PBS 30'-5 at the output. The polarized optical signals are output from the optical output port 02', and the non-polarized optical signals are output from an optical output port 01'. At the same time, other optical signals are introduced at optical input port 02. There is a one-to-one correspondence between the frequency of the RF signal frequency, namely the frequency of the SAW, and the wavelength of the optical signal to be polarized, under a constant temperature. In other words, it is possible to select the wavelength of an optical signal to be output by varying the RF signal frequency.
When the AOTF 30' is used as in an OADM, the optical input port 01 is usually used as the main input port, the optical input port 02 as the add light input port, the optical output port 01' as the main output port and the optical output port 02' as the drop light input port. When the RF signal is supplied to the transducer, it is possible to simultaneously add and drop optical signals having a wavelength corresponding to the frequency of the RF signal. Further, if a plurality of RF signals of different frequencies are supplied to the electrodes, it is possible to select optical signals having a plurality of wavelengths respectively corresponding to those RF signals. That is, the foregoing construction is very effective for use with an OADM filter that simultaneously adds and drops optical signals having a plurality of wavelengths. The AOTF is bi-directional in principle, and to replace the input port with the output and vice versa will maintain the same operation.
The AOTF 30' shown in FIG. 24 may be used in the wavelength demultiplexing/multiplexing device shown in FIG. 22. However, the construction shown in FIG. 22 requires two AOTFs, and moreover, requires two RF signal sources and two driving circuits to drive the two AOTFs. Accordingly, the device becomes complicated, and this is a problem.